I made it through the summer with no algae problems with my new liner, but on returning from a 9 day trip on 10/17 thought I saw one fairly bright patch of yellow about the size of a saucer by the steps. It looked too bright to me for mustard algae and since we have a lot of yellow flowers near the pool thought it might be from them so I brushed and vacuumed the pool. Before I left in the morning of the trip the chlorine level was 4ppm (I maintained my pool at 4-6 PPM all summer) and I added 1 jug of the 121 oz. 8.25% concentrated chlorine bleach which normally raises the level 4 ppm (25000 gallons in ground cya 60 (as of today but around 55 most of the time), ph 7.5, TA 100). I also stacked 7 of the 3" chlorine tablets in the skimmer.
When I returned home the chlorine level was > 10 (I can't remember the level exactly) and by today it is still 7 ppm. Last weekend I saw a few small disjointed patches (< size of a saucer) that looks like it may be green algae and there was about an 18/6/6 rectangle on the wall of the pool by the steps. A day or two ago I saw the few spots again on the bottom. I didn't address the problem previously because I wasn't sure what it was and it didn't seem to be growing much and I just didn't have enough consecutive time to go into shock mode. I have the following question.
I'm familiar with the algae shocking procedure but what I'm confused about is closing the pool under these conditions. Currently my water temperature is 65 and I know that slows down algae growth but also slows down the chlorine use rate. I recall reading awhile back that before closing, the pool should be shocked and let the chlorine return to normal levels before covering. For the last 4 days my chlorine level has dropped by <= 1 ppm and my CC is 0 and the water is clear as a bell. However, since I've been home it's been cloudy more days than not which probably slows the use rate. So I think that may make it hard to use the overnight drop test as well as to see of there is any chlorine use the same day I shock it. So does that means I'm going to have to strictly go by a visual test which may take days?
Since it takes so long for the chlorine to drop I want to use the minimum amount of chlorine. Pool school says with a CYA of 60 it requires a level of 26 ppm but the pool calculator says 18 with 34 for mustard. If I do 26 it may be Christmas before I get back to normal shock levels. There is little danger of pipes freezing by then but I'd like to get it done before all the leaves fall in and they are just starting to fall.
I know that last year that I shocked the pool to about 18 (no algae) and covered the pool the next day and when I took the cover off, the underside of the cover was bleached white and the remaining part of the liner that wasn't bleached was now bleached. I wasn't concerned because I knew I was replacing the liner in the spring.
Does anyone know the upper limit of chlorine that will prevent bleaching in a covered pool?
So on the one hand I don't want to bleach out my new liner and suffer death at the hands of my wife but I also don't want to open it up with an algae bloom which has happened to me before. Currently my plan is to shock it to 18 tomorrow, check the overnight drop test and let it drop to about 5-6 and cover it when the temperature drops to 60, checking for visible sgns of algae. If algae reappears, I'm not sure what I will do. I'm fairly sure that I want to uncover it when the temperature is below 60, correct? Any suggestions on the best course of action?
Thanks for your help.